any or several small, transparent crustaceans of the order Cladocera, having the body covered by a bivalve shell from which the head and antennae extend.
cla·doc·er·an (klə-dŏs'ər-ən) n. Any of various small, mostly freshwater crustaceans of the order Cladocera, which includes the water fleas. adj. Of or belonging to the order Cladocera.
[From New Latin Cladocera, order name : Greek klados, branch + Greek keras, horn; see ker-1 in Indo-European roots.]